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Weather school: Rare and unusual clouds

Canada’s Northwest Passage is currently open, albeit with some patchy ice

In this article we will continue our study of clouds by looking at several additional terms that can be used to describe and help identify them, and we’ll also look at some rare or unusual cloud types. Before diving into this topic, I want to do a quick update on the state of Arctic sea ice as we approach the yearly ice minimum.

According to the National Snow and Ice Data Center (U.S.), as of Sept. 1, Arctic sea ice extent stood at 4.26 million square kilometres, the second-lowest extent for that date in the satellite record that began in 1979. The Arctic Ocean has lost about 3.15 million square km of ice in August over the last four decades, which works out to be about an 11 per cent decline per decade. With the minimum soon occurring, it looks like this summer’s minimum will come in as the second or third lowest on record. The northern sea route (along Russia) is wide open and has been for some time, while the Northwest Passage through Canada is now open, but with some patchy ice.

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Back to clouds. Along with our 10 primary cloud types we can add a number of descriptive words to help describe all of the variations of clouds that we can see. The table you’ll see here is a list of some (but not all) of these terms.

These descriptive words are usually added to the end of our main cloud type name. For example, if we have stratus clouds that have a ragged or torn appearance, we could call them stratus fractus clouds. If we see several cumulus clouds together growing vertically, we could call them cumulus castellanus clouds.

Some of these cloud descriptor terms may be used on their own to identify clouds. A couple of examples of this would be mammatus clouds, which we occasionally see with thunderstorms, or cumulonimbus clouds and lenticular clouds, usually seen near mountains. In the case of mammatus clouds, which can be associated with several of our main cloud types, they should be described technically with the main cloud type first and then the term mammatus should follow it (i.e. cumulonimbus mammatus) but most of the time you will simply see them reported or described as mammatus clouds.

Lenticular clouds are named in a similar way. While we should name them as altocumulus lenticularis clouds or cirrus lenticularis, for example, we tend to simply group them all together as lenticular clouds. One of the reasons for this is that these cloud types are fairly rare. The lenticular clouds form when a smooth airflow rises over a barrier — such as a mountain — which causes a wave to form in the airflow. This is very similar to the wave formed when water flows over a rock. The part of the air stream that is forced upward by the barrier cools and a cloud will form taking the shape of the wave. Then as the air sinks back down again the air warms and the water in the cloud evaporates and the cloud disappears. This results in a lens-shaped cloud that appears to stand stationary.

Mammatus and lenticular clouds are not the only rare cloud types; we also have nacreous clouds, or what’s often called Mother-of-Pearl clouds. These clouds can be seen in northern countries when the sun is low on the horizon during the middle of winter. They are best visible in the early dawn or after dusk, because they are a very high cloud and are found up in the stratosphere (rather than the troposphere), and at these times of the day they reflect sunlight from below the horizon, making them visible.

If you are interested in all the different, unusual, and rare cloud types that are out there or if you just want to see some cool cloud pictures (along with some other interesting pictures), check out this website.

In the next article, with many regions already experiencing a frost, I think it will be time to do our annual look at frost and frost-free days.

About the author

Co-operator contributor

Daniel Bezte

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the U of W. He operates a computerized weather station near Birds Hill Park.



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